3l6 CHELONIA CHAP. 



ends of the iliac bones ; but since these find a much more effective 

 support in the shell, the distal ends of the sacral vertebrae 

 fuse with the eighth, or so-called last, pair of costal plates. 



The neural arch of the ninth vertebra rests upon its 

 centrum; but the neural arches of the other trunk -vertebrae, 

 although long, rest upon two centra ; retaining, like the ribs, 

 their original intercentral position ; and in most cases the 

 neuro-central siitures remain throughout life. The atlas and 

 the last cervical vertebra deserve special attention. In many 

 tortoises, e.g. Trionyoi, Clemmys, Testmlo, the three constituent 

 parts of the atlas, namely, the neural arch, the centrum, and 

 the intercentrum or first pair of united basiventralia, do not 

 ankylose, but remain loosely connected ; and the first centrum, 

 instead of forming an odontoid process, remains movably attached 

 to the second centrum, although it sometimes carries, and fuses 

 with, the second intercentral piece. In other tortoises, e.g. 

 Platemys and Chelys, however, all the parts of the atlas co-ossify 



and form a complete, solid vertebra which 

 articulates by a concavo-convex joint with 

 the' centrum of the second vertebra. The 

 normal numl)er of cervical vertebrae is 

 eight in all Chelonians. The first spinal 

 nerve issues between occiput and atlas, 

 all the others behind the neural arches of 

 Fig 62.-1, The complete ^j -^^ vertebrae. The last, or eighth cer- 



atlas ot an adult Trionijx '^ 



hurum. The second basi- vical, owing to the retractility of the neck, 

 Z^^SrlT^ forms elaborate joints; its centre fits with 

 first centrum, which, not a knolj iuto a cup ot the ninth, and its 



being fused with the second , i r i i , ,,^^i 



centrum, is not yet an post-zygapophyscs form broad, cur^ cd 

 odontoid process. 2, The articulating concave facets for the recep- 



coniijlete atlas of an adult , . „ ^ , ^ . , p , , 



Triomjx gancjeticus, still tion of the anterior zygapophyses ot the 

 typically temnospondylous. ^^q^ ninth Vertebra. In the Trionychidae 



3, The first and second cer- . i i . i 



deal vertebrae of an adult the zygapophyses are most elaborate, and 



Platemys. 4, The complete ^|^gy .^\q^q articulate with the ninth ver- 



at\as of a Chelys Jimbnata. , ., , i • • i 



tebra, while the centra do not join, but 

 remain, or rather become, separated by partial resorption. In 

 the Chelonidae, in conformity with the non-retractile and short 

 neck, all the cervical joints are much reduced. 



The skull (cf Fig. 54, H, I, K, p. 280) agrees fundamentally 

 with tiiat of Sphenodon and of the Crocodilia, but it is 



